Nitrate accumulates in recirculation aquaculture systems (RAS) when water exchange is reduced and denitrification technologies are excluded from the process flow. Although nitrate is generally less toxic to fish than ammonia and nitrite, several studies have reported sublethal effects to RAS-produced species. During an onsite study, rainbow trout exhibited toxicity symptoms when nitrate-nitrogen (NO3-N) was maintained at 90 mg/L. On the contrary, another onsite study found that post-smolt Atlantic salmon were unaffected by 100 mg/L NO3-N. Follow up research is underway to determine if higher NO3-N concentrations (150–200 mg/L) negatively impact post-smolt Atlantic salmon growth, health, and welfare in low exchange RAS.
To begin the 7-month study, 35 all-female Atlantic salmon (282 g mean weight) were surgically implanted with biomonitors (Star-Oddi, Iceland) that continuously record fish heart rate. These data are expected to elucidate any stress response triggered by accumulating nitrate. Salmon receiving biomonitors were kept in recovery tanks for one month and then divided into six replicate RAS along with equal numbers of untagged fish from the base population. After stocking, feed loading rates were gradually increased causing NO3-N to accumulate to 80 mg/L. Peristaltic pumps were then used to repeatedly dose concentrated sodium nitrate in three RAS resulting in a gradual increase to 150 mg/L NO3-N (Fig. 1). Sodium sulfate was added similarly in three control RAS to balance ionic concentrations while maintaining 50–100 mg/L NO3-N. At the time of abstract submission, salmon in both treatments were ~1.4 kg and adjustments were underway to increase NO3-N to 200 mg/L. Salmon growth, health, welfare scores, and heart rate data will be reported at the meeting.
Knowledge gleaned from this research could redefine the upper nitrate threshold for post-smolt Atlantic salmon production in RAS. The findings could also lead to reduced water use requirements and minimized necessity for inclusion of denitrification technologies.